1. Field of the Invention
The invention relates in general to sensors which use rare-earth iron garnets as sensor elements, and in particular to sensors which use such garnets to rotate linearly polarized light for measurement of magnetic fields, electrical current, or temperature fluctuations.
2. Related Art
The rapid technology expansion in the military and commercial industries has dramatically increased the need for small, highly robust methods of monitoring parameters such as magnetic fields, electrical current flow, and temperature. Fiber optic sensors capable of monitoring these parameters have been under research and development for the better part of 30 years and have recently seen a significant improvement in performance as new advances in materials and manufacturing are made. Unfortunately, the practical implementation aspects of many of the prior inventions leave much to be desired in terms of ease of use.
The need to monitor magnetic fields and electrical current is enormous. Power companies are concerned with power losses all the way from the generation point to the final destinationxe2x80x94the consumer. Additionally, power companies bill consumers based upon consumption; unfortunately, they still use monitoring technology that predates most of modern memory. Loss of the capability to monitor current flow results in potentially large losses in revenue. Power electronic system manufacturers, those who are responsible for converting standard electrical mains to varying levels of AC and DC, are constantly faced with limitations in sensor and control technology in the implementation of their designs. Their goals of increased efficiency (lower heat), smaller designs for a given power density, and increases in switching frequency are pushing the limits of conventional current and magnetic sensing technology. Coupled with this industry is an older industryxe2x80x94motor controls. Personnel involved with motor controls are constantly seeking better methods of increasing output efficiency through optimization of run-time parametersxe2x80x94all of which are derived from magnetic and current sensors. Physical limitations in existing sensor capabilities are restricting large advances in hybrid-motor development, which has had a measurable impact on the development of automotive hybrid engines.
Optical sensors are poised to revolutionize the sensor industry through their intrinsic advantages. In many instances the bandwidth of optical sensors is limited only by signal processing constraintsxe2x80x94not the sensor material as is the case with many conventional sensors. Optical sensors are often immune to electromagnetic interference (EMI) noise; hence, they do not require specialized shielding in high-noise environments. This results in a smaller transducer cable with significantly less weight added to the entire assembly, an added benefit for industrial and aerospace/aircraft applications. It also removes the need for localized signal conditioning equipment to be positioned close to the monitoring point, resulting in potential savings in overall systems cost. Because optical sensors are typically small devices as compared to their conventional sensor counterparts, they have the potential to fit into smaller areas or be integrated into existing designs with little modification. The dielectric nature of optical fiber gives it an intrinsic isolation in signal measurement in high-voltage or -current applications, which is a considerable benefit to the electrical power and power semiconductor industry. Additionally, optical sensors are inert, which allows their use in potentially explosive environments. Finally, optical sensors can be remotely positioned from the signal processing equipment, an advantage that has tremendous benefits for the aerospace, aircraft, and automotive industries.
Certain materials change the polarization state of incident light in the presence of a magnetic field. This property, known as the Faraday effect, is widely used in the fiber optic telecommunications field, specifically to prevent reflected light energy from coupling back into a light source and changing source parameters such as frequency or power output. In sensor systems that exploit the Faraday effect, a sensor assembly is placed into a magnetic field. By monitoring the rotation of the incident polarization state, a direct measurement of the magnetic field intensity can be inferred.
Optical fiber is one material that exhibits a small Faraday effect. Based upon this, many inventions have been disclosed which measure the amount of current flowing through a conductor. By wrapping multiple turns of optical fiber around the conductor and applying Ampere""s Law (increasing the path length), the amount of current can be directly measured. Unfortunately, this sensor method is often impractical in many applications because it is not feasible to interrupt power by disconnecting the conductor, installing the fiber coil assembly, then reconnecting the conductor.
Certain crystalline materials, known as rare-earth iron garnets (REIGs), exhibit a much larger Faraday rotation per unit magnetic flux density than their fiber optic waveguide counterparts. These crystals are synthetic and typically (but not exclusively) have a general formula equation
R3xe2x88x92xBixFe5xe2x88x92xAyO12
where R is from the family of elements comprising Y, La, Ce, Pt, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, A is from the family of elements comprising Ga, Sc, Al, or In, and 0.3 less than =x less than =2.0 and 0.0 less than =y less than =1.0. A number of methods exist to grow these materials and range from the flux method to the Czochralski method to liquid-phase epitaxial growth (LPE). The preferred method of growing the crystals is subjective and is somewhat dependent upon the operating wavelength of interest; no preferred method is implied or required for the present invention as sensor materials from both LPE and flux methods were implemented.
Sensor housings which use a REIG material have historically been designed in transmission modexe2x80x94linearly polarized light enters one side of the crystal, travels the length of the crystal, and then exits the distant endface. Optics are required at the distant endface to either directly quantify the amount of rotation, or to couple the energy back into a waveguide so that it can be remotely optically processed. Although this configuration can work in many applications, axial alignment of the optical components during manufacturing renders these configurations difficult to mass-produce. This topology also introduces size constraints in the fiber optic embodiment of the inventionxe2x80x94the separate lead-in and lead-out fiber requirement increases the overall size of the sensor probe due to increased bulk optics, often making the monitoring of magnetic flux in applications such as switched-reluctance systems very difficult.
U.S. Pat. No. 4,563,639 (1986) to Langeac discloses a temperature and/or electrical intensity measuring apparatus based upon the Faraday effect. The sensor is an optical fiber wound in the form of a solenoid and is connected to a polarized light source as well as a beamsplitter and photodiodes. Through selective twisting of fiber, problems due to birefringence in the lead-in and lead-out fibers are overcome.
U.S. Pat. No. 5,463,316 (1995) and U.S. Pat. No. 5,493,222 (1996) to Shirai et al. discloses a reflection type magneto-optic sensor head. The head assembly specifically uses an integrated polarizer, a Faraday rotator comprised of a (111) bismuth-substituted iron garnet single crystal film, and a reflecting film, all of unitary construction such that each are in contact with the other. Also disclosed is another embodiment of this basic concept using rectangular prisms. The light launch/recovery system uses a laser light source, an input collimating lens, half mirror, multimode polymer-clad optical fiber, an output collimating lens, and a single photodiode. No discussion with respect to signal processing is made.
U.S. Pat. No. 5,483,161 (1996) to Deeter et al. discloses a magnetic field sensor utilizing high-permeability magnetic flux concentrators with a high-permeability magneto-optic sensing element to increase measurement sensitivity. The sensor is used in transmission mode (separate transmit and receive fibers) and utilizes convex lenses to collimate the polarized wavefront.
It is therefore an object of the invention to provide an improved sensor for measuring polarization rotation of optical wavefronts.
It is a further object of the invention to provide a fiber optic reflectance-type system that does not require a separate optical path to recover the measurand.
In a preferred embodiment, the invention provides sensors that use rare-earth iron garnets as sensor elements to measure magnetic fields, electrical current, or temperature fluctuations. The invention may be provided in the form of a bulk-optical system preferably aligned in the order of a laser light source, a polarizer, a chopper, a first-surface mirror, an anti-reflection coating, a Faraday rotator material, a reflecting film, a polarization beam splitter, and two low-noise detectors. The invention may also be embodied in a fiber optic system preferably comprising a light source pigtailed to optical fiber, an opto isolator, a fiber optic coupler, optical fiber, a grin lens, an anti-reflection coating, a Faraday rotator material, a semiconductor substrate, a reflecting film, a polarization beam splitter, and two low noise detectors. A further embodiment provides a semiconductor sensor system using a rare earth iron garnet sensor element.